The invention relates to a dryer or heat exchanger for drying or SSP pre-treatment (heating without adhesion) of PET granulate or PA or PEN granulate. It can also be used for drying other granular bulk materials.
Methods and devices for crystallising and aftercondensing PET in solid phase are well known. Aftercondensing of PET in solid phase (SSP) occurs at temperatures exceeding 200xc2x0 C. for several hours in suitable reactors or dryers. The problem consists of achieving the highest possible throughput and quality of material at minimal expenditure. The process applied inter alia uses even heating of the PET and at the same time removes reaction products.
The use of roof dryers or column dryers for SSP treatment of PET granulates is well known. Thus DE-A-4300913 describes a pre-heater with roof-shaped baffles to achieve an even gas flow in the reverse flow. At the same time the baffles are also designed to achieve an evening out of the product flow (relative movement and prevention of sticking) as well as a reduction of the bulk pressure. Also known are shaft reactors with net-like baffles (DE-C-2753543) among others. The manufacturing expense for such baffles is high.
CH-PS 314655 describes a device for de-dusting and classifying granular materials, with the product falling down being evenly de-dusted by means of an ascending airstream. The air enters the device laterally beside or below a displacer.
Also known are dryers for nuts, cocoa beans and similar which essentially comprise an upper section with two heating zones and a lower section with cooling zones.
The product passes the dryer/roaster from top to bottom in a narrow channel whose longitudinal sides comprise grates covered by wire mesh. The width of the channel is adjustable and the grating is removable. The small design is a disadvantage as it does not allow large throughput with long dwell time. From the point of view of process technology, simple upscaling is not possible. Such a dryer/roaster is air-operated and not airtight. With the products to be treated, the problem of agglomeration does not arise; in addition the products are comparatively large and their bulk density is low.
It is thus the object of the invention to improve the above-mentioned state of the art by developing a dryer for granulate, in particular of PET, PEN or PA for their thermal pre-treatment, which dryer makes possible a bulk flow with short to extended dwell time and large design height.
This object is met in that in a dryer which in itself is of conventional design and comprises a circular or rectangular cross-section, a channel is arranged whose side walls are elements such as perforated panels or similar. The width of the channel is preferably 25 to 40 cm at a length of up to approx. 3.5 m and a height of up to or exceeding 15 m. A displacer and/or deflection elements may be arranged in the lower region of the channel.
This makes it possible to achieve a high gas throughput without easing off and thus a short heat-up time, combined with a compact and economical design size.
It is necessary to achieve a finely distributed gas flow, i.e. a good through flow of the product with compartmentalised gasification (cross-flow, inverse flow) against the product stream. Dwell time of the products is usually 1-2 hours. To achieve a good flow through the product, including a relatively large to minor loss in air pressure, the holes in the perforated panels have a nominal diameter of only a few millimeters. Alternatively, instead of using actual perforated panels, panels with fine, slot-shaped apertures (covering the channel surface entirely or partly) are used.
The outer casing is rectangular or round (resistance to pressure hammer; maintenance considerations). Further baffles parallel to the gas flow can be arranged in the channel, e.g. panels alternately pointing downwards, to prevent the granulate from sticking due to lack of movement.